Opiates play a major role in the management of pain. Most opioids used clinically act through the mu opioid receptor, a G-protein-coupled receptor encoded by the Oprm gene. Despite a common receptor class through which they act, the pharmacology of mu opioids vary widely among patients. Twenty-five years ago we proposed multiple mu opioid receptors based upon a series of receptor binding and traditional pharmacological studies. The mu opioid receptor has been cloned and termed MOR-1. We have now identified 25 different splice variants of MOR-1 in the mouse, 10 in humans and 8 in rats, confirming our earlier suggestion of multiple mu opioid receptors. In the mouse, the splicing is complex due to the presence of two independent promoters located over 10 kb apart. The exon 1 promoter generates 16 different variants while the exon 11 promoter is responsible for 9. In addition to traditional, full length 7 transmembrane receptors, a number of truncated forms have been identified. The objective of this proposal is to continue our studies exploring MOR-1 and its actions, trying to understand the roles of these new splice variants. This will include receptor binding and functional assays. Studies on the truncated variants will be undertaken in an effort to assess their potential significance and function. A major component of this work will address the issue of the proteins associated with the receptors in the membrane. This receptor complex, which likely includes G-proteins and other regulatory proteins, may play a major role in understanding the functional differences observed among the full length variants and possibly even the activity of the truncated forms. We also will explore potential new splice variants. ORL1 is a member of the opioid family and is selective for its endogenous ligand OFQ/N. Expressed alone, ORL1 binds OFQ/N with very high affinity and is insensitive to traditional opioids. However, ORL1 dimerizes with MOR-1 to form a complex with unique pharmacological properties. Specifically, co-expression with MOR-1 renders ORL1 binding sensitive to a range of mu opioids. We will continue our studies of OFQ/N binding sites identified in brain and explore putative receptors for the OFQ/N-related peptides. Together, these studies should give a better understanding of the actions of opioids and their pharmacology.